Neoplasia, the pathological process by which tumors develop, necessarily involves unregulated, or at best misregulated, cellular growth and division. The molecular pathways that regulate cellular growth must inevitably intersect with those that regulate the cell cycle. The cell cycle consists of a cell division phase and the events that occur during the period between successive cell divisions, known as interphase. Interphase is composed of successive G1, S, and G2 phases, and normally comprises 90% or more of the total cell cycle time. Most cell components are made continuously throughout interphase; it is therefore difficult to define distinct stages in the progression of the growing cell through interphase. One exception is DNA synthesis, since the DNA in the cell nucleus is replicated only during a limited portion of interphase. This period is denoted as the S phase (S=synthesis) of the cell cycle. The other distinct stage of the cell cycle is the cell division phase, which includes both nuclear division (mitosis) and the cytoplasmic division (cytokinesis) that follows. The entire cell division phase is denoted as the M phase (M=mitotic). This leaves the period between the M phase and the start of DNA synthesis, which is called the G1 phase (G=gap), and the period between the completion of DNA synthesis and the next M phase, which is called the G2 phase (Alberts, B. et al., Molecular Biology of the Cell, Garland Publishing, Inc., New York & London (1983), pages 611-612.).
Progression through different transitions in the eukaryotic cell cycle is positively regulated by a family of master enzymes, the cyclin-dependent kinases (reviewed by Sherr, C. J., Cell 73:1059-1065 (1993)). These holoenzymes are composed of two proteins, a regulatory subunit (the cyclin), and an associated catalytic subunit (the actual cyclin-dependent kinase or CDK), the levels of which vary with different phases of the cell cycle (Peters, G., Nature 371:204-205 (1994)). Both cyclins and CDKs represent molecular families that encompass a variety of genetically related but functionally distinct proteins. Generally, different types of cyclins are designated by letters (i.e., cyclin A, cyclin B, cyclin D, cyclin E, etc.); CDKs are distinguished by numbers (CDK1, CDK2, CDK3, CDK4, CDK5, etc.; CDK1 is a.k.a. CDC2).
CDK-cyclin D complexes regulate the decision of cells to replicate their chromosomal DNA (Sherr, Cell 73:1059-1065 (1993)). As cells enter the cycle from quiescence, the accumulation of CDK-cyclin D holoenzymes occurs in response to mitogenic stimulation, with their kinase activities being first detected in mid-G1 phase and increasing as cells approach the G1/S boundary (Matsushime et al., Mol. Cell. Biol. 14:2066-2076 (1994); Meyerson and Harlow, Mol. Cell. Biol. 14:2077-2086 (1994)). The cyclin D regulatory subunits are highly labile, and premature withdrawal of growth factors in G1 phase results in a rapid decay of CDK-cyclin D activity that correlates with the failure to enter S phase. In contrast, removal of growth factors late in G1 phase, although resulting in a similar collapse of CDK-cyclin D activity, has no effect on further progression through the cell cycle (Matsushime et al., Cell 65:701-713 (1991)). Microinjection of antibodies to cyclin D1 into fibroblasts during G1 prevents entry into the S phase, but injections performed at or after the G1.fwdarw.S transition are without effect (Baldin et al., Genes & Devel. 7:812-821 (1993); Quelle et al., Genes & Devel. 7:1559-1571 (1993)). Therefore, CDK-cyclin D complexes execute their critical functions at a late G1 checkpoint, after which cells become independent of mitogens for completion of the cycle.
In mammals, cells enter the cell cycle and progress through G1 phase in response to extracellular growth signals which trigger the transcriptional induction of D-type cyclins. The accumulation of D cyclins leads to their association with two distinct catalytic partners, CDK4 and CDK6, to form kinase holoenzymes. Several observations argue for a significant role of the cyclin D-dependent kinases in phosphorylating the retinoblastoma protein, pRb, leading to the release of pRB-associated transcription factors that are necessary to facilitate progression through the G1.fwdarw.S transition. First, CDK-cyclin D complexes have a distinct substrate preference for pRb but do not phosphorylate the canonical CDK substrate, histone H1 (Matsushime et al., Cell 71:323-334 (1992); Matsushime et al., Mol. Cell. Biol. 14:2066-2076 (1994); Meyerson and Harlow, Mol. Cell. Biol. 14:2077-2086 (1994)). Their substrate specificity may be mediated in part by the ability of D-type cyclins to bind to pRb directly, an interaction which is facilitated by a Leu-X-Cys-X-Glu pentapeptide that the D cyclins share with DNA oncoproteins that also bind pRb (Dowdy et al., Cell 73:499-511 (1993); Ewen et al., Cell 73:487-497 (1993); Kato et al., Genes & Devel. 7:331-342 (1993)). Second, cells in which pRb function has been disrupted by mutation, deletion, or after transformation by DNA tumor viruses are no longer inhibited from entering S phase by microinjection of antibodies to D cyclin, indicating that they have lost their dependency on the cyclin D-regulated G1 checkpoint (Lukas et al., J. Cell. Biol. 125:625-638 (1994); Tam et al., Oncogene 9:2663-2674 (1994)). However, introduction of pRb into such cells restores their requirement for cyclin D function (Lukas et al., J. Cell. Biol. 125:625-638 (1994)). Third, pRb-negative cells synthesize elevated levels of a 16 kDa polypeptide inhibitor of CDK4, "p16.sup.InK4a " (a.k.a. "InK4a-p16" or simply "p16"), which is a member of a recently discovered class of cell cycle regulatory proteins (Nasmyth and Hunt, Nature 366:634-635 (1993); Peters, G., Nature 371:204-205 (1994)) and which is found in complexes with CDK4 at the expense of D-type cyclins during G1 phase (Bates et al., Oncogene 9:1633-1640 (1994); Serrano et al., Nature 366:704-707 (1993); Xiong et al., Genes & Devel. 7:1572-1583 (1993)). The fact that such cells cycle in the face of apparent CDK4 inhibition again implies that D-type cyclins are dispensable in the Rb-negative setting.
The InK4 gene family ("InK4" signifies Inhibitors of CDK4) is known to include at least three other low molecular weight polypeptides, InK4b-p15, induced in human epithelial cells treated by transforming growth factor-.beta. (TGF-.beta.) (Hannon, G. J., and Beach, D., Nature 371:257-261 (1994)), InK4d-p19 (Hirai, H., et al., Mol. Cell. Biol. 15:2672-2681 (1995)) and InK4c-p18 (Guan et al., Genes & Develop. 8:2939-2952 (1994); Hirai, H., et al., Mol. Cell. Biol. 15:2672-2681 (1995)). InK4d-p19 and InK4c-p18 are described in detail in Ser. No. 08/384,106, filed Feb. 6, 1995, which is hereby incorporated by reference.
Members of the InK4 family are typically composed of repeated ankyrin motifs, each of about 32 amino acids in length. All known members of the InK4 family act to specifically inhibit enzymatic activities of D-type cyclin-dependent kinases such as CDK4 and CDK6. Unlike other universal CDK inhibitors, such as p21.sup.Cip1/Waf1 (El-Deiry et al., Cell 75:817-825 (1993); Gu et al., Nature 366:707-710 (1993); Harper et al., Cell 75:805-816 (1993); Xiong et al., Nature 366:701-704 (1993)) and p27.sup.Kip1 (Polyak et al., Genes & Devel. 8:9-22 (1994); Polyak et al., Cell 78:59-66 (1994); Toyoshima and Hunter, Cell 78:67-74 (1994)), the InK4 proteins selectively inhibit the activities of CDK4 and CDK6, but do not inhibit the activities of other CDKs (Guan et al., Genes & Devel. 8:2939-2952 (1994); Hannon and Beach, Nature 371:257-261 (1994); Serrano et al., Nature 366:704-707 (1993)).
Like many CDK inhibitors (CKIs) (Nasmyth and Hunt, Nature 366:634-635 (1993)), InK4 family members negatively regulate progression through the mammalian cell cycle, in part in response to anti-proliferative extracellular signals. The InK4 proteins, by inhibiting the activities of a specific class of the D-type cyclin-dependent kinases (i.e., CDK4 and/or CDK6), arrest cell cycle progression in G1 phase and thus prevent cells from replicating their chromosomal DNA. Thus, in contradistinction to the positive regulation of D-type cyclin synthesis by growth factors, extracellular inhibitors of G1 progression can negatively regulate the activity of D-type cyclin-dependent kinases by inducing InK4 proteins.